AIR INTERFACE

By:

Eng\ WALEED EL SHAFAI

AIR INTERFACE

GSM 900

0.8 0.9 1.0 1.1 1.6 1.7 1.8 1.9 2.0 GHz

GSM 1800 GSM 1900

200 kHz

f

Carrier n

Carrier (n+1)

• GSM bands

AIR INTERFACE

Uplink and downlink

• GSM Frequency Bands

System P-GSM 900 E-GSM 900 GSM(DCS)

1800 GSM(PCS) 1900

Uplink (MS BS) Downlink(BS MS)

890 – 915 MHz 935 – 960 MHz

880 – 915 MHz 925 - 960 MHz

1710 – 1785 MHz 1805 - 1880 MHz

1850 – 1910 MHz 1930 - 1990 MHz

Wavelength 33 cm 33 cm 17 cm 16 cm

Bandwidth 25 MHz 35 MHz 75 MHz 60 MHz

Duplex distance 45 MHz 45 MHz 95 MHz 80 MHz

Carrier separation 200 kHz 200 kHz 200 kHz 200 kHz

No. of carriers 124 174 374 299

Channel rate 270.8 kbps 270.8 kbps 270.8 kbps 270.8 kbps

AIR INTERFACE

AIR INTERFACE

- TDMA in GSM

AIR INTERFACE

• TDMA frame

AIR INTERFACE

• TDMA frame structure

3 1 1 3 57 57 26

AIR INTERFACE

• TDMA frame structure

-Bit rate of the radio carrier is 270.833 Kbps

-Bit duration =1/270.833=3.69 µsec

-One time slot =148 bits+8.25 guard bits=156.25 bits

-Time slot duration =156.25x3.69 µsec= 0.577 msec

-Frame duration=0.577x8= 4.615 msec

AIR INTERFACE

• Guard band is used for :

- burst alignment errors

- time dispersion on the propagation path

- Time required for smooth switch on-off in transmitter

1 2 3

• The numbering scheme of the time slots is staggered by 3 time slots to remove necessity for MS to send and receive in the same time

4 5 6

1 2 3 4 5 6 0

0

AIR INTERFACE

• Types of channels

1. Physical channels

2. Logic channels

AIR INTERFACE

• Types of logic channels

1. Traffic channels

2. Control channels

AIR INTERFACE

• Types of Traffic channels

1. Full rate

- 13 Kbps data rate

2. Enhanced full rate

- 12.2 Kbps data rate

3. Half rate

- increase network capacity

13 Kbps 9.8 Kbps

12.2 Kbps 10.6 Kbps

5.6 Kbps 5.8 Kbps

11.4 Kbps

22.8 Kbps

Data

Redundancy

AIR INTERFACE

• Types of control channels

• Broadcast control channel

• Dedicated control channel

• Common control channel

AIR INTERFACE

Common Control Channels (CCCHs) Transfer control information between all MSs and BTS. Necessary for call origination and paging.

Logical ch. Function BTS MS

Paging CHannel (PCH)

Downlink

•Alert the MS of an incoming call

Transmits paging message to indicate an incoming call The paging message contains MS identity.

Listens to PCH regularly. When identifies its identity, it responds.

Random Access CHannel (RACH)

Uplink

•Used by MS to request access to the network.

Receives request from MS for a signaling channel.

Answers paging message on RACH by requesting signaling channel.

Access Grant CHannel (AGCH) Downlink

•Used by BS to inform MS about the channel to use.

Assigns a SDCCH signaling channel to MS.

Receives a signaling channel assignment SDCCH.

AIR INTERFACE

Dedicated Control Channels (DCCHs) Carry messages between MS and network. SDCCH is used for call setup, update, authentication

Logical ch. Function BTS MS

Stand-alone Dedicated Control Channel (SDCCH)

UL+DL

•Exchange signaling information in uplink and downlink.

Switches to SDCCH. Call set-up is performed in idle mode. BSC assigns a TCH.

Switches to SDCCH. Call set up is performed. MS receives TCH assignment [carrier + time slot]

Slow Associated Control Channel (SACCH)

UL+DL

•Conveys power control and timing information in downlink. •Conveys link quality reports in uplink.

Instructs MS about: Transmit power Time advance.

Sends measurements of its BTS and neighboring BTSs during a call.

Fast Associated Control Channel (FACCH)

UL+DL

•Steals TCH to carry handover and channel reassignment.

Transmits handover information.

Transmits handover request..

The Broadcast Channels (BCHs) Is transmitted by BS all the time. Monitored by MSs periodically (every 30 sec). All BCHs are downlink.

Logical channel Function BTS MS

Frequency Correction Channel (FCCH)

Supply MS with reference freq.

Transmits a carrier frequency.

Identifies BCCH carrier

synchronizes with the frequency.

Synch. Channel (SCH)

For TDMA frame synchronization .

Transmits frame number and BSIC.

Synchronizes with frame structure.

Broadcast Control Channel (BCCH)

•Carries parameters needed to identify and access network. •It is transmitted at constant power at all times and checked by all MSs.

Broadcasts cell information such as

LAI, maximum output

power needed

Receives LAI May update MS sets its output power level.

AIR INTERFACE

AIR INTERFACE

• Adaptive Frame Alignment The BS-MS delay depends on distance. If BS–MS separation= 10km, Propagation delay=33 sec. Long guard interval is inefficient. GSM uses adaptive time alignment system whereby BS transmits timing advance at the MS. This instructs the mis-aligned MS to transmit earlier

TDMA frame

Propagation delay

TS0 TS2 TS1

B: Far away from BS

B A

A: Close to BS

AIR INTERFACE

Rayleigh fading is frequency-selective.

BS&MS hop from one frequency to other.

Frequency hopping is optional.

Rate of hopping =1/4.615msec=217 hop/sec.

Slow Frequency Hopping